Gaseous-fuelled internal combustion engines are becoming known in the automotive industry as a possible alternative to diesel fuelled engines. Replacing diesel fuel with gaseous fuels provides the potential for reducing emissions as diesel fuel burns with higher concentrations of pollutants. Gaseous fuels are generally defined herein as fuels that are in gaseous phase at standard pressure and temperature conditions. “Standard pressure and temperature” are defined herein to be an absolute pressure of 1 bar (14.5 psi) and 0 degrees Celsius (32 degrees Fahrenheit). Natural gas is a mixture of combustible gases and its exact composition varies depending on the source, but it is ordinarily primarily methane. Other gaseous fuels include ethane, propane, and other flammable gaseous hydrocarbon derivatives as well as hydrogen and mixtures thereof. Compared to conventional liquid fuels, the disclosed gaseous fuels are generally cleaner burning, can be produced from renewable sources and are generally readily available. For example, natural gas is one of the most abundant hydrocarbon fuels available today.
A challenge, however has been to substitute diesel fuel with gaseous fuels while maintaining the performance of diesel-fuelled engines including the power and efficiency of a diesel engine and at the same time further reducing the engine emissions.
For one type of internal combustion engines, gaseous fuel is injected directly into the combustion chamber of the internal combustion engine, when the piston is near top dead center causing the fuel to burn in a diffusion combustion mode or in a stratified mode and in such direct injection engines gaseous fuel can be ignited by a small amount of pilot fuel, for example diesel fuel that enables a complete and efficient combustion of the fuel. Such dual fuel engines have been described in previous patents by the applicant, for example in several United States patents (U.S. Pat. No. 7,463,967, U.S. Pat. No. 8,028,676, U.S. Pat. No. 8,469,009 and U.S. Pat. No. 8,555,852).
Because gaseous fuels are compressible fluids, it takes more energy to compress and raise the pressure of gaseous fuels compared to liquid fuels. Therefore in the past one of the main factors in determining the maximum gaseous fuel injection pressure has been to use the minimum pressure required to force gas into the combustion chamber at full load. A lower injection pressure for gaseous fuels has been generally desired because employing higher injection pressures increases the parasitic load on the engine system. Since conventional diesel fuelled engines which have been converted to a dual fuel operation have a maximum cylinder pressure during the compression stroke of around 200 bar, a gaseous fuel injection pressure higher than around 200 bar, for example an injection pressure of up to about 300 bar (30 MPa or 4,350 psi) has been sufficient for overcoming the cylinder pressure with enough energy to disperse the gaseous fuel within the combustion chamber and to inject the desired amount of fuel for achieving an efficient combustion, as described in applicant's U.S. Pat. No. 8,095,294. This patent recognizes that changes in the cylinder pressure can influence the speed at which the valve needle of the fuel injector opens and closes and thereby can influence the amount of fuel being injected into the combustion chamber and addresses this problem by correcting the amount of time that the fuel injector stays open (the “pulse width”) to thereby adjust the fuelling amount. Injecting the gaseous fuel at higher pressures is not investigated.
Co-owned U.S. Pat. No. 7,162,995 describes a method of injecting gaseous fuel into a high pressure direct injection internal combustion engine comprising selecting an injection pressure that results in an under-expanded gaseous fuel beyond and proximate to the injector nozzle and mentions injecting gaseous fuel into the combustion chamber at an injection pressure greater than 30 MPa (300 bar) with a pressure ratio which is a function of the injection pressure and the cylinder pressure in excess of a ratio beyond which gas flow is choked. Such a pressure ratio provides the pressures necessary for a supersonic flow into the combustion chamber of a natural gas fuelled direct injection engine while avoiding interference with the cylinder walls and piston. As described in this patent, in most cases the injection pressure would be in excess of 10 MPa (100 bar) and will be selected such that it avoids the gas jet interference with the cylinder walls or piston.
While gaseous fuels are generally cleaner burning than conventional diesel engines, tailpipe emissions from gaseous-fuelled engines can be further improved to reduce the levels of particulate matter, hydrocarbons and NOx by applying an after-treatment to the gases exhausted from the engine. Such an after-treatment system can be complex and increases the cost of the overall engine system.
While some of the existing prior art has mentioned injecting gaseous fuel into the combustion chamber of an internal combustion engine at injection pressures higher than 300 bar, no known prior art has further investigated the engine performance and efficiency at such high pressures to identify a preferred range for the injection pressure and for the pressure ratio between the injection pressure and the peak cylinder pressure that would allow a better engine performance and efficiency while improving engine emissions.
Therefore there is still a need to investigate such engine parameters for further reducing the emission levels in gaseous-fuelled engines and for reducing the complexity and cost of the after-treatment system while maintaining or preferably improving the engine efficiency and performance.